Conventionally, ethylene and propylene are produced via steam cracking of paraffinic feedstocks including ethane, propane, naphtha and hydrowax. An alternative route to ethylene and propylene is an oxygenate-to-olefin (OTO) process. Interest in OTO processes for producing ethylene and propylene is growing in view of the increasing availability of natural gas. Methane in the natural gas can be converted into for instance to methanol or dimethylether (DME), both of which are suitable feedstocks for an OTO process.
In EP2018359 A1, an OTO process is described wherein an oxygenate such as methanol or DME is provided with an C4+ olefinic co-feed to a reaction zone together comprising a suitable zeolite conversion catalyst and converted to ethylene and propylene. During start-up of the process of EP2018359 A1, an external C4+ olefinic co-feed is supplied to the process to provide the C4+ olefinic co-feed required for the reaction. At a certain stage the external olefinic co-feed is at least partially replaced by recycling C4+ olefins obtained from the effluent of the reaction zone and the process moves in to a steady state or normal operation.
The use of a feedstock comprising oxygenates together with an olefin co-feed is typical for OTO process that use a zeolite-comprising catalyst. During start-up of the process, no reaction product is yet produced and therefore external source of olefins needs to be provided. Typically, this is a C4 hydrocarbon fraction obtained from for instance a steam cracker or a catalytic cracker. As the process operation moves from the start-up phase into normal operation, sufficient reaction product is produced and the supply of external olefins is gradually reduced, while the recycle of olefins, in particular C4 olefins, from the reaction product to the OTO zone is increased to satisfy the olefin demand of the OTO process.
A disadvantage of using an external source of olefins as described herein above is that these external olefins are only required during start up of the process. For instance at a refinery site, this would require rerouting of olefin-comprising hydrocarbon fractions away from their designated purpose. Not only does this present logistic challenges, it may also cause disruption of processes that normally rely on these olefin-comprising hydrocarbon fractions as feedstock. Typically, an external source of olefins contains diolefins. These diolefins can cause additional coke formation during start-up. During normal operation of the process, these diolefins are typically removed by a selective hydrogenation unit. However, these units are generally not yet operational during start-up. By using a tert-alkyl ether during start-up, instead of the external source of olefins, no diolefins are present. An additional disadvantage is that these olefin comprising hydrocarbon fractions typically comprise significant concentrations of paraffins. Paraffins do not react in the OTO process, and therefore only act as a diluent. Although this may be acceptable during normal operation it is undesired during start-up.
There is a need in the art for a more efficient method to conduct the start-up of such an OTO process.